Will the US freeze F35 delivery to Turkey?

The United States could soon freeze preparations for delivering F-35 fighter jets to Turkey, officials told Reuters, in what would be the strongest signal yet by Washington that Ankara cannot have both the advanced aircraft and Russia's S-400 air defence system.

The United States is nearing an inflection point in a years-long standoff with Turkey, a NATO ally, after so far failing to sway President Tayyip Erdogan that buying a Russian air defence system would compromise the security of F-35 aircraft.

"The S-400 is a computer. The F-35 is a computer. You don't hook your computer to your adversary's computer and that's basically what we would be doing," Katie Wheelbarger, acting assistant secretary of defence for international security affairs, told Reuters.

While no decision has been made yet, U.S. officials confirmed that Washington was considering halting steps now underway to ready Turkey to receive the F-35, which is built by Lockheed Martin Corp.

"There (are) decisions that come up constantly about things being delivered in anticipation of them eventually taking custody of the planes," said Wheelbarger.

"So there's a lot of things in train that can be paused to send signals to them (that we're serious)," she added, without detailing those steps.

If Turkey was removed from the F-35 programme, it would be the most serious crisis in the relationship between the two allies in decades, according to Bulent Aliriza, director of the Turkey project at the Center for Strategic and International Studies.

The strains on ties between Washington and Ankara already extend beyond the F-35 to include strategy in Syria, Iran sanctions and the detention of U.S. consular staff.

"This (the F-35 standoff) is really a symptom, not a cause of the problem between the two countries," Aliriza said.

Many U.S. officials, speaking on condition of anonymity, worry that Turkey is drifting away from NATO and watch improving relations between Ankara and Moscow with concern. The prospect of Russian contractors or officials on Turkish bases that also are home to the F-35 is unfathomable to many U.S. officials.

The tensions could further escalate. If Ankara goes ahead with the Russian deal, Turkey also could face U.S. sanctions.